Within the petrochemical industry, there are many instances where a very detailed analyses of a process feed or product is needed for the purpose of making business decisions, planning, controlling and optimizing operations, and certifying products. Such a detailed analysis is referred to as an assay, a crude assay being one example thereof.
Traditionally, when a crude oil is assayed, it is distilled in two steps. A method such as ASTM D2892 (see Annual Book of ASTM Standards, Volumes 5.01-5.03, American Society for Testing and Materials, Philadelphia, Pa.) is used to isolate distillate cuts boiling below approximately 650° F. (343° C.). The residue from this distillation is further distilled using a method such as ASTM D5236 to produce distillate cuts covering the range from 650° F. to approximately 1000-1054° F. (343° C. to approximately 538-568° C.) and a vacuum residue cut. At a minimum, cuts corresponding to typical products or unit feeds are typically isolated, including LPG (Initial Boiling Point to 68° F.), LSR (68-155° F.), naphtha (155-350° F.), kerosene (350-500° F.), diesel (500-650° F.), vacuum gas oil (650° F. to 1000-1054° F.), and vacuum residue (1000-1054° F.+). Each distillate cut is then analyzed for elemental, molecular, physical and/or performance properties. The specific analyses conducted depend on the typical disposition of the cut. The data derived from these analyses will typically be stored in an electronic database where it can be mathematically manipulated to estimate crude qualities for any desired distillation range. For example, commercial crude assay libraries are available from Haverly Systems Inc., and HPI Consultants Inc., both of which provide tools for manipulating the data, as does Aspentech Inc. Assay data is published by Crude Quality Inc., by Shell Oil Company, and by Statoil. The property versus distillation temperature data is typically fit to smooth curves that can then be used to estimate the property for any desired distillation cut. Crude assays that are generated via the distillation of the crude oil are herein referred to as “wet” crude assays to distinguish them from assay generated by other means.
The intent of the crude assay is to generate data representative of current crude oil quality for use in making business decisions, planning, controlling and optimizing operations, and certifying products. This representative assay is herein referred to as a Recommended Assay. These Recommended Assays are utilized to determine appropriate product slates for a given crude oil and identify refineries that are suitable for processing such crude oils.
Crude oil is not a homogenous entity. Physical and chemical characteristics of a crude oil change during the production life of the field. These characteristics may also change based upon the location of the crude oil within the field. In addition, crude oils from different fields are often blended together to produce a particular grade of crude oil that is commercially offered for sale. Changes in production volumes, field maintenance, new wells being brought onstream, or changes in a given fields crude oil quality over time can have an additional and often dramatic impact on the quality of a given crude oil grade. When such changes occur, the Recommended Assay may no longer be representative of the current crude oil quality.
Historically, crude oil monitoring has usually limited to a handful of easily and quickly measured properties including API gravity, sediment and water, (BS&W), salt and sulfur. These properties are usually referred to as Inspection Properties. Frequently, the only measurements made are API gravity and water, which are required to properly determine the amount of oil being sold. While these two properties can provide some indication of changes in crude oil quality, these two properties are extremely limited and more detailed monitoring and tracking of crude oil is desirable to make informed crude oil purchase and refining business decisions. More detailed characterizations have typically involved a laboratory distillation based assay which is relatively expensive, and can take several weeks to months to complete. Performing an assay of this type on cargo purchases to monitor and value crude oil quality changes would be impractical due to the time delay in obtaining the data. Real time monitoring and valuation of crude oil is desirable to make informed crude purchase and refining business decisions.
A given crude oil grade may not exhibit changes in API gravity even when the yield structure may vary dramatically. API gravity changes are typically accompanied by a shift in yields, such that a lower API gravity typically indicates an increase of heavier boiling materials. However, situations can occur where yield structure changes do not exhibit associated changes in the gravity. An example would be where naphtha boiling range components (68-375° F.) may decrease, with an associated increase in diesel range material (375-530° F.), accompanied by a shift of resid material boiling in the 1050+° F. range decreasing with an increase in gas oil material (530-1050° F.). While the overall yield structure resulting from these yield changes would be significantly different, the API gravities may not exhibit large changes. Yield changes could have a material impact on crude oil value that in this case would not be evident from the API gravity measurement. As such, additional evaluation of other properties is needed to determine whether or not a particular crude oil is appropriate for the production of the desired product slate or the processing in a particular refinery.
Properties in addition to gravity are also used to evaluate whether a given crude oil is economically attractive or whether it can be processed in a given refinery. Sulfur, neutralization number, or metals are examples of properties that may vary with time and can impact the ability of a given refinery to process a crude oil. For example, not all refineries are capable of processing crude oils that have a high sulfur content. Similarly, not all refineries are capable of processing heavy crude oils. API gravity provides no indication of a change in these qualities, but changes in these values would affect the crude oil's economic value.
Presently, there are well over 1000 unique commercially available crude oil grades. This presents a logistical issue with monitoring crude oil quality, detecting significant deviations from expected quality, and properly evaluating these changes. It is desirable to have the ability to quickly and efficiently obtain a more detailed characterization of the crude oil and monitor the properties in an organized manner in order to provide more insight for crude oil valuation. There is a need for an automated system that generates the characterization data, detects quality deviations, and triggers notifications for follow-up actions to ensure that changes in crude oil properties are identified and reflected in business decisions is desirable such that suitable crude oils are used to produce desired product slates and processed in the desired refinery.
The current state of the art for monitoring crude oil quality varies from simple plots of time series data of easily measured inspection properties as gravity, to application of correlative techniques to laboratory measurements. These time series data are tracked on a large number of crude oils which have commercial interest and are available globally through an internal company intranet website. Many of the laboratory tests are very time consuming taking weeks or longer to generate useful results.